Due to its high volume yield, optical microlithography appears to remain the method of choice for patterning microelectronic and other micro-scale chips for the foreseeable future. However, high yields can only be maintained through optical microlithography if the size of the process window is maintained large. The process window characterizes permitted variances of the dose and focus relative to the nominal dose and focus used to imprint mask features on the wafer. The nominal dose and focus are typically determined empirically by printing a critical dimension mask shape on a substrate, e.g. semiconductor wafer, and verifying that the critical dimension is achieved. Since in the mass production of chips, it is never possible to guarantee that the nominal dose and focus are maintained, the circuit layout and mask generation processes must be designed to provide an adequately large process window. In such manner, the circuit resulting from the lithographic patterning will meet performance goals, despite variations in the dose and the focus.
Resolution enhancement techniques (RETs) are among known techniques used to increase the process window associated with the lithographic process. The generation and placement of sub-resolution assist features (SRAFs) (also referred to as scattering bars in some literature) on photomasks is one of the most popular RETs in the deep sub-micron (below 100 nm) lithography technology for manufacturing chips having very large scale integration (VLSI). SRAFs are additional rectangular shapes that are placed on masks along the length or the extent of main mask shapes in deep sub-micron lithography. The SRAFs do not print themselves but help to improve the process window for wafer printing by improving the depth of focus of the mask shapes. FIG. 1 is a top down view illustrating the placement of SRAFs 11, 12, 13 and 14 along sides of a main mask shape 10. FIG. 1 illustrates a particular type of SRAF known as “same tone”. The shapes on a photomask can have either one of two tones: clear (permitting the light to pass) and opaque (blocking the light). The main mask shapes can be either clear or opaque, such as depends on whether the photoresist of the wafer to be patterned is a positive or a negative resist. Same tone SRAFs have the same tone as the main mask shape that they assist. Thus, in the same tone arrangement illustrated in FIG. 1, the mask shape 10 and the SRAFs 11-14 are all either clear or opaque features.
Instead of having the same tone, SRAFs can have the opposite tone as the main mask shape that they assist. When the SRAF has the opposite tone than the main features, they are referred to as “opposite tone” SRAF. In this case, an SRAF can be considered as being holes in the main feature. FIG. 2 is a top down view illustrating an example of a main mask shape 20 which is assisted by the opposite tone SRAFs 21-23. Hereinafter, as illustrated in FIGS. 1 and 2, SRAFs are illustrated as shapes of a mask in addition to the main mask shapes.
In addition to masks having same tone SRAFs and masks having opposite tone SRAFs, both same tone and opposite tone SRAFs may be present together in the same mask. This type of SRAFs is referred to as dual tone SRAFs. An example of dual tone SRAF is shown in the plan view provided in FIG. 3. As shown therein, same tone SRAFs 26 and 28 are placed external to the main mask shapes 30, 32 and 34, along the length of the main mask shapes. Opposite tone SRAFs 31, 33, and 35 are also provided internal to each of the main mask shapes 30, 32 and 34.
Different nomenclature is sometimes used in literature instead of that used above. For example, if SRAFs occupy a clear region in the mask they are referred to as clear SRAFs and if they occupy a dark region in the mask they are referred to as dark SRAFs or opaque SRAFs. Often opaque SRAFs are referred to as positive tone SRAFs and clear SRAFs are referred to as the negative tone SRAFs.
Hereinafter, the term SRAF will be used to refer to all such SRAFs, whether they have the same tone or the opposite tone as the main mask shapes that they assist, unless reference is made specifically to SRAFs having the same tone or opposite tone.
Conventionally, SRAFs are placed on a mask in accordance to a rule table used by lithographic engineers. The rule table dictates the following things: a) the number of SRAF to be placed along an edge of a mask shape or a fraction of an edge of a main mask shape; b) the distance of the SRAF from the edge of the mask shape it is assisting; c) the width of the SRAF; and d) the distance between two SRAFs, if there is more than one SRAF along an edge.
The rule table that is used for SRAF placement decides the above five items on a one dimensional criterion of the allowable space between two neighboring opposite-facing main mask shapes.
In addition to that, there are several mask manufacturing constraints that decide the location, and the size of the SRAF. These mask manufacturing rules may decide the following items: a) the minimum area of each SRAF; b) the minimum width of each SRAF; c) the minimum length of each SRAF; and d) the minimum distance between an SRAF and a main feature or another SRAF: The distance can be characterized by design rule checking (DRC) directives. Some examples of DRC directives are: corner-to-corner checks and end-to-end checks.
According to commonly used methods, SRAFs are placed on a mask as follows: a) placing SRAFs using the rule table; b) cleaning up, i.e. reshaping and/or deleting SRAFs that can create catastrophic conditions, such as printing of the SRAF on the wafer (SRAFs are intended not to be printed on the wafer, but rather to help in better print the main features on the wafer); and c) cleaning up, i.e. reshaping, SRAFs having mask rule violations that remain on the mask after step b). An SRAF exhibits a mask rule violation when the SRAF shows any one or more of the following: any side (defined as any bounding line segment) of the SRAF has less than minimum width according to mask manufacturing constraints, or any side of the SRAF is placed so close to another mask shape (either main mask shape or another SRAF) that there is less than a minimum spacing between that SRAF and the other mask shape.
A conventional method of placing of SRAFs is illustrated in the flow diagram of FIG. 4. As shown therein, the conventional method begins with the generation of the main shapes of the mask from a set of design data defining a circuit layout, as shown at block 50. A set of rule tables for the generation of SRAFs and a set of mask manufacturing rules are also provided at this time, as also shown at block 50. From the main shapes and the SRAF rule tables, SRAFs are generated for the mask, as shown at block 52. Next, as shown at block 54, an individual SRAF is considered. The individual SRAF is checked against the mask manufacturing rules, at block 56. That is, it is determined whether the individual SRAF violates minimum spacing rules relative to any other shape of the mask, and whether the shape of the individual SRAF violates minimum width rules.
If the decision is that the SRAF satisfies the mask rules, i.e. the decision is “yes” that the SRAF does not have mask rule violations, then the SRAF is allowed to remain on the mask. At block 60, it is then determined whether any other SRAF of the remains to be considered. If there remains an unconsidered SRAF, then another SRAF of the mask is considered in turn, as shown at block 57. However, if the decision is “no”, i.e., that the SRAF does show mask rule violations, then the SRAF is deleted from the mask, as shown at block 58. Thereafter, it is determined whether any other SRAF of the remains to be considered, at block 60. Again, if any unconsidered SRAF remains, then another SRAF of the mask is considered in turn, as shown at block 57. On the other hand, once all of the SRAFs of the mask have been considered, data representing the SRAFs are output, at block 62.
Unfortunately, the algorithm according to the prior art is quick to delete SRAFs upon discovery of mask rule violations. Stated another way, the algorithm does not favor preservation of SRAFs, but rather, preserves only those SRAFs which do not have mask violations when viewed in relation to the initial main mask shapes. As a result, many a mask edge is often left unassisted by an SRAF, and the process window is consequently diminished.
The challenge of mask clean-up, i.e. the process of removing mask rule violations, is exacerbated by the tendency for the spacings between the SRAF and the edges of mask shapes to vary in width. In regions where the spacing width has a high rate of variation, the length of the SRAFs and the total mask area of the SRAFs becomes small. Unfortunately, by the prior art method described above with respect to FIG. 4, small SRAFs are eventually deleted from the mask during the mask clean-up process as violating minimum width and/or minimum spacing rules.
An example of this problem of mask clean-up according to the prior art method is illustrated in FIGS. 5A and 5B. FIG. 5A illustrates a set of main shapes 70, 72 of a mask and corresponding opposite tone SRAFs 71 and 73, prior to performing any mask clean-up process. The main shapes 70, 72 extend from first edges 74 to second edges 75 in a horizontal direction 78 across a main surface of the mask. The SRAFs 71, 73 also extend from first edges 74 to second edges 75 in the horizontal direction 78. However, the irregular outline of the main shapes 70, 72 in areas 77 near edges 75 causes the SRAFs 71, 73 in those areas 77 to be jogged.
FIG. 5B illustrates the resulting main shapes 70, 72 and SRAFs 81, 83 after mask clean-up. During clean-up, each portion of the SRAFs 71, 73 is considered individually as to whether it satisfies mask rules 56, according to the process illustrated in FIG. 4. As a result of the clean-up process, the SRAFs 81, 83 are pulled back a desirable distance from the edges 74, leading to better printing of the main shapes 70, 72. However, near the edges 75, the areas 77 where the SRAFs 71, 73 are jogged have mask violations. Following the clean-up process, the resulting SRAFs 81, 83 do not appear at all in the areas 77, leaving the edges 75 of the main shapes 70, 72 unassisted by the SRAFs 81, 83. This is clearly an undesirable result, as the process window is substantially decreased.
FIG. 6 illustrates the results of mask clean-up according to a modified processing method. Referring again to FIG. 5A, in such method, known as “partial smoothing,” the jogged portions 77 are first smoothed, i.e. straightened, prior to testing them against mask manufacturing rules for minimum feature width and spacing. Then, after the subsequent mask clean-up process, more of the once jogged portions 77 of the SRAFs 71, 73 remain as the final SRAF shapes 91, 92, 93 and 94 of the mask. In this case, reasonable assist coverage is achieved while satisfying the mask manufacturing constraints rules.
Another example of the results of the mask clean-up process is illustrated in FIGS. 7 and 8. As shown in FIG. 7, the main mask shapes 101, 102 are more complicated than in previous examples, and have non-uniform spacing with respect to each other and within each main mask shape. As a consequence, the opposite tone SRAFs 103 and 104 which are generated to assist the main mask shapes are very small. After mask clean-up, as shown in FIG. 8, the resulting main mask shapes 101, 102 are left unassisted. This again is undesirable, as the process window for printing the main shapes is narrowed. It would be desirable instead for SRAFs to remain sufficiently after the mask clean-up process in such size that they actually assist the lithography process window as intended. Such situation is illustrated in FIG. 9 in which the resulting SRAFs 114 are still large enough and in sufficient quantity to genuinely help to maintain the process window.
The problem of missing SRAFs as demonstrated in all the above examples frequently arises from the non-uniform spacings in which SRAFs are placed. In many cases, the nonuniformity arises from dents, jogs, or channels in the main mask shapes that the SRAFs are intended to assist, or in some combination thereof. However, these small variations, in truth, should not cause SRAFs to be deleted or significantly shortened from the mask, as is so often the case according to the mask clean-up methods described above with respect to FIGS. 1-9.
Accordingly, it would be desirable to provide a method for systematically smoothing, i.e. removing non-uniformities of, main mask shapes prior to that are intended to assist them.
It would further be desirable to provide a method for generating reasonably sized SRAFs for assisting the smoothed main mask shapes.